Hydraulic fracturing is a stimulation treatment which consists of propagating fractures in rock layers by the introduction of a pressurized treatment fluid. The treatment fluid is pumped at high pressure into the hydrocarbon bearing area of a wellbore that extends into the target reservoir. The high pressure fluid when hydraulically injected into the wellbore causes cracks or fractures which extend outwardly and away from the wellbore into the surrounding rock formation.
Depending on the nature of the reservoir and the particular rock formation, acid, chemicals, sand or other proppants are selectively mixed into the treatment fluid to improve or enhance the recovery of hydrocarbons within the formation.
There have been a number of recent developments with respect to wellbore treatment tools including the development of tubular fracturing strings for staged well treatment. Such fracturing strings are predicated on creating a series of isolated zones within a wellbore using packers. Within each zone there are one or more fluid ports that can be selectively opened from the surface by the operator. A common mechanism includes a sliding sub actuated by a ball and seat system, the movement of which is used to open fluid ports. By sizing the seats and balls in a complimentary manner, increasingly larger balls may be used to selectively activate a particular sliding sub allowing the operator to stimulate specific target areas.
Further development and refinement has resulted in fracturing strings having multiple fluid ports within each isolated zone. The seats and balls are sized such that one ball may be used to actuate a series of sliding subs within an isolated zone or a series of sliding subs in different isolated zones. This is achieved using seats that expand or deform to allow the ball to pass. The ball is deployed from the surface, travels down the well bore, and becomes lodged on the deformable seat to form a temporary seal. The fluid pressure on the ball and seat actuates the sliding sub from its initial, first position into its second position, and in the process opens the fluid port. With continued fluid pressure, the seat eventually deforms, allowing the ball to pass through the seat and down to the next sliding sub, where it actuates the next sliding sub in the same manner. The last or lowest seat in the isolated zone is sized such that the ball will not pass, thus forming a seal to prevent the flow of treatment fluid to any lower zones that may have already been actuated and treated. The use of multiple fluid ports allows multiple stages within the isolated zone to be stimulated with one surface treatment. This type of fracturing method is generally termed limited entry fracturing.
When using a fracturing string with multiple deformable seats and a single ball, as described above, the operator may encounter difficulties in fracturing the lower regions of the formation within the isolated zone. The reason for this problem is that the seats are designed so that greater fluid pressure is needed to push the ball past the lower situated seats than the higher situated seats. This greater fluid pressure may be sufficient to force the fluid from the fracturing string into the well bore and to fracture the formation surrounding the already opened higher fluid ports. This results in a loss of fluid which is counterproductive to increasing fluid pressure in the fracturing string. Accordingly, the operator may be unable to achieve sufficient fluid pressure to push the ball past the seats and actuate the sliding subs situated in the lower regions of the formation. Even if the operator can achieve sufficient pressure to activate the subs in the lower regions of the formation, the pressure may still be sub-optimal for stimulating the lower regions of the formation. Prior art solutions have enjoyed limited success and are relatively complicated.
More recent developments in fracturing have suggested the use of rupture disks or burst disks within the fracturing tools. For example, U.S. Patent Publication No. 2011/0192613 to Garcia et al., and U.S. Patent Publication No. 2015/0260012 to Themig describe fracturing tools having fluid ports covered with temporary port covers which are designed to gradually tear or erode to an open position with the use of erosive and/or corrosive treatment fluids. This can cause problems with the fracturing operation, since initial pumping rates to gradually erode or corrode the fluid covers are low and less predictable until the fluid cover is fully eroded to open the fluid ports. Low flow rates of fracturing fluids are generally not desirable since the treatment fluid is carrying sand, and “sanding off” or plugging of the fluid ports and other equipment can occur at low flow rates. As well, there is less precision in directing the treatment fluid to the desired area to be fractured while the treatment fluid is being pumped at low flow rates.
Applicant's earlier patent application, U.S. Patent Publication No. 2014/0102709 to Arabskyy, describes a fracturing tool and method in which the fluid ports of a fracturing tool are closed by a burst plug which is designed to allow treatment fluid to flow through the fluid port in response to a prescribed threshold hydraulic pressure level of the treatment fluid. Particularly for limited entry fracturing processes, this fracturing tool and method allows for greater reliability and precision for operators, since the opening pressure of the fluid ports is a prescribed threshold pressure that can be set considerably higher than the pressure needed to shift the sliding subs in a series of fracturing tools. Thus, the operator can be confident that the fluid ports are not opened below the prescribed threshold pressure of the burst plugs, thus preventing the escape of treatment fluids from the fluid ports within an isolated zone until the treatment fluid pressure has been raised to the level required for hydraulic fracturing.
More recent patents and patent applications describing fracturing tools with burst plugs include PCT Patent Publications WO 2015/095950, WO 2015/117221 and WO 2015/117224, all to Arabsky et al., and U.S. Pat. No. 9,228,421 to Kent et al.
In fracturing operations, reliable opening of the flow ports in the fracturing tools is important. Operators prefer reliable and predictable flow restrictions (i.e., flow area and diameter) at the flow ports when pumping fluid downhole. Erosion of the fluid ports, whether or not closed with burst plugs, remains problematic in fracturing operations, particularly in view of the erosive and/or corrosive nature of the treatment fluids.